Massive Multiple-Input Multiple-Output (MIMO) is one of important characteristics in Time-Division Long-Term Evolution (TD-LTE) technical evolution (4.5G), and characterized in flexible network deployment, easiness to select a site, enhanced coverage, lower interference, and an extended capacity.
With massive MIMO, the average spectrum efficiency of a cell is improved by installing hundreds of antennas (e.g., 128 or more antennas) in a base station As illustrated in FIG. 1, when there are more and more antennas of the base station, channels of UEs are increasingly orthogonal to each other, interference between the UEs are disappearing, and the signal to noise ratio of each UE is improved effectively due to a significant array gain, thus supporting data transmission by more UEs on the same time-frequency resources, and improving the average spectrum efficiency of the cell.
On the other hand, another advantage of 3D-MIMO over legacy antennas is that MIMO in the horizontal and vertical directions is realizable with a two-dimension array of antennas.
In legacy MIMO, there are typically fixed weighting phases for respective dipoles in the vertical direction of respective antenna elements, and thus fixed downward-inclination angles, and respective antenna elements in the horizontal direction are weighted dynamically for dynamic MIMO in the horizontal direction, thus resulting in 2D MIMO. When different dipoles or antenna elements in the horizontal and vertical directions throughout the array of antennas are weighted in phase and amplitude dynamically and controllably, then MIMO in both the horizontal direction and the vertical direction (3D MIMO) is realizable, thus further improving the number of available spatial dimensions of MIMO, so as to improve the spectrum efficiency of a wireless communication system in a space with one more dimension as illustrated in FIG. 2.
With massive MIMO, a weight of broadcast beam-forming can be adjusted dynamically according to the locations of UEs, and of course, this can only be applicable to a Physical Downlink Shared Channel (PDSCH) demodulated using a UE-specific Reference Signal (URS), and for a Physical Downlink Control Channel (PDCCH), a Physical Control Format Indicator Channel (PCFICH), a Physical Downlink Shared Channel (PDSCH), etc., demodulated using a Common Reference Signal (CRS), since the CRS is a cell-level parameter, in order to apply to all the UEs in the coverage area of the cell, the weight of broadcast beam-forming can not be designed for some UE, but the directivity and the coverage area of a broadcast beam shall apply to all the UEs in the cell.
Taking 64 antennas as an example, in order to guarantee a coverage of 65 degrees in the horizontal and vertical directions, weights of broadcast beam-forming of antennas available from some antenna manufactures are as follows: amplitudes of respective antennas in the vertical direction at 445 degrees are [0.45, 1, 0.9, 0.4, 0.4, 0.9, 1, 0.45], and their phases are 10, 90, 165, −120, −190, 175, 90, −101; and amplitudes of respective antennas in the horizontal direction are [0.35, 0.65, 1, 1, 0.35, 0.65, 1, 1], and their phases are [0, −172, 2, 12, 0, −172, 2, 12], where weights of broadcast beam-forming at −45 degrees are the same as those at +45 degrees. As illustrated in FIG. 3, an antenna in a darker color represents an antenna in the polarization direction of +45 degrees, and an antenna in a lighter color represents an antenna in the polarization direction of −45 degrees.
From the Kronecker products of the weights of broadcast beam-forming in the horizontal and vertical directions, amplitudes of a first column of eight antennas in the vertical direction are [0.1575, 0.35, 0.315, 0.14, 0.315, 0.35, 0.1575], amplitudes of a second column of eight antennas are [0.2925, 0.65, 0.585, 0.26, 0.26, 0.585, 0.65, 0.2925], amplitudes of a third column of eight antennas are [0.45, 1, 0.9, 0.4, 0.4, 0.9, 1, 0.45], and amplitudes of a fourth column of eight antennas are [0.45, 1, 0.9, 0.4, 0.4, 0.9, 1, 0.45]. As can be apparent from the amplitudes of these weighted antennas, many of them are far below 1, and there is such a serious loss of power that the performance of the channels demodulated using a CRS will be degraded seriously.
In summary, only the preconfigured weights of broadcast beam-forming is used to cover the entire coverage area of the cell in the prior art, thus in order to enable the broadcast beam to cover all the locations in the coverage area of the cell, there is such an inevitable loss of energy of the broadcast beam, and the loss may become significant as the number of antennas is increasing.